Whether studying the core of our sun or the inside of a fusion reactor, scientists need to determine how energy flows in plasma. Scientists use simulations to calculate the flow. The simulations rely on the classical thermal transport model. Despite over 50 years of research, an ad hoc multiplier is often required. Without it, the simulation doesn’t match real-world observations. Now, a team devised a way to measure energy flow and determined why the models need the multiplier. Further, the team’s new approach lets them quantitatively test simulations.